The problem appears to be that when you upload and embed an image, for some reason Blogger is omitting the
http:from the beginning of the image URL. This means that all the image URLs now start with just
//. This is perfectly valid (it's defined in RFC 1808) and is often referred to as a scheme relative URL. What this means is that the URL is resolved relative to the scheme (http, https, ftp, file, etc.) of the page in which it is embedded.
I'm guessing this changes means that blogger is looking at switching some blogs to be served over
http. Leaving the scheme off the image URL means that the image will be served using the same scheme as the page regardless of what that is. The problem though is that the code that blogger uses to generate the post feeds seem to ignore images that don't start
http:meaning that no thumbnails are generated.
For now the easy fix is to manually add the
http:to the image URLs (or at least of the image you want to use as the thumbnail for the post). Of course it would be better if blogger just fixed their code to spot these URLs properly and include them in the post feed.
Updated 18th November 2014: It looks as if this problem has been resolved as I've just tried posting on one of my other blogs and the
http:bit is back.